Soul of the Duelist (SOD) introduced new level monsters to the game which require the lower LV version of itself to be summoned. This set is also notable for being the first set where Secret Rares have been eliminated as a rarity the first set to introduce Ultimate Rare cards. Highly sought after cards include the "Horus" set, "Armed Dragon" set, "Mobius the Frost Monarch", and "Hallowed Life Barrier."